The present invention relates to methods for heat treating metal parts or workpieces and more particularly, to the carburizing and carbonitriding of steel parts in vestibule furnaces.
The carburization of steel parts is a well known process wherein a "case" is imparted at and below the part surface for the purpose of substantially increasing the carbon content so that such parts may be hardened upon quenching. Typically, steel parts are carburized in a vestibule furnace which is essentially comprised of at least two chambers. An outer chamber which is generally referred to as the furnace vestibule is provided to enable atmosphere coverage of the quench which may be an atmosphere and/or an oil quench. Depending upon the particular furnace construction utilized, workpieces may be charged directly into a work chamber and then removed into a vestibule for a subsequent atmosphere or oil quench. Alternately, workpieces may be loaded in a vestibule, passed to the work chamber and then returned to the same vestibule for quenching. In a continuous furnace inlet and outlet vestibules are provided before and after a hot and/or work zones.
Upon the introduction of an appropriate carrier gas, typically an endothermic or purified exothermic gas, which may be enriched with a quantity of natural gas, a door to the work chamber is opened and the metal parts to be carburized are then transferred to the work chamber which has been previously brought to the necessary temperature. In typical carburizing processes, endothermic gas which is essentially comprised of 40% nitrogen, 40% hydrogen and 20% carbon monoxide with minor or trace amounts of carbon dioxide and water vapor, is supplied to the work chamber and vestibule at a flow rate sufficient to continuously sweep these chambers and substantially prevent the introduction of atmospheric oxygen into the vestibule. In order to assure that a sufficient quantity of a carbon source is present within the atmosphere of the work chamber, the endothermic gas is enriched with a flow of natural gas. It has been found, however, that in order to adequately carburize steel parts in such a vestibule furnace, substantial amounts of natural gas are consumed as the generation and use of endothermic gas in a carburizing furnace requires natural gas or other hydrocarbon source such as propane. Thus, for each 100 cu.ft. of endothermic gas, approximately 45-50 cu.ft. of natural gas are consumed in producing "endo" gas and when enriching natural gas is utilized, as much as 10-20 cu.ft. of additional natural gas are required for each 100 cu.ft. of endothermic gas. Therefore, it is clear that a relatively high and virtually unavoidable consumption of natural gas inherently occurs in the course of conventionally carburizing steel parts in vestibule furnaces.
One rather plain consequence of the steadily increasing demand for hydrocarbon fuels has been reflected as a severe and even critical shortage of natural gas. Presently, numerous industrial users of natural gas are facing sharp curtailment in the quantities supplied if not outright interruption of natural gas flows. Accordingly, many industrial users of natural gas such as heat treating plants in general and steel carburizing facilities in particular, are of necessity forced to substantially reduce natural gas consumption. Accordingly, it is an imperative for those heat treating facilities with vestibule furnaces that substantial reductions in natural gas consumption must be achieved to enable continued carburization of steel parts. Additionally, alternative carburizing techniques must result in the adequate carburization of steel parts at a cost which is economically comparable to the present cost of carburizing steel parts by the aforedescribed process relying upon endothermic and natural gas in order to assure that the metallurgical benefits resulting from carburization in general are commercially justifiable.
In addition to carburizing steel in vestibule furnaces, it is known to conduct this process in a furnace type which is not provided with separate inlet/outlet and work treating zones. Such a furnace is normally referred to as a "pit" furnace and with the addition of appropriate auxiliary equipment such as conduits, filters, meters, and compressors or the like, a furnace atmosphere may be removed from the pit furnace and recirculated in combination with a reduced flow of a carburizing source such as natural gas with an overall reduction of natural gas being obtained in comparison with a similar furnace utilizing a carrier gas such as endothermic gas described above. Such a pit furnace is illustrated in Davis II, U.S. Pat. No. 3,397,875 and although reductions of the consumption of carburizing materials can be realized, integral quenching of carburized steel parts is incompatible with pit furnaces.
As mentioned previously, conventional carburizing processes conducted within vestibule furnaces rely upon a flow of endothermic gas to the work chamber to control the flow of decarburizing agents such as atmospheric oxygen, etc., into this chamber as well as to provide an adequate purge of the furnace vestibule to maintain the oxygen concentration therein below the lower combustible limit. In addition, the amount of natural gas supplied to the work chamber (in addition to the natural gas required for generation and combustion of the endothermic gas) must be sufficient to overcome the decarburizing effect of any contaminants such as oxygen, water vapor CO.sub.2 or the like which either leak into, or are generated by reactions within the work chamber as well as those which are contained in the carrier gas and, obviously, to satisfy the carbon demand of the work load. In certain heat treating processes, such as described in U.S. Pat. No. 3,467,366 an inert gas such as nitrogen is supplied to the vestibule of a furnace to enable isolation of the work chamber from atmosphere. However, neither in this prior art nor in other industrial carburizing processes presently known, is there any recognition of the advantages to be obtained from supplying such inert gas to a vestibule and essentially only a gaseous carbon source to a work chamber which may have been purged of deleterious gases during a heat-up period as will now be described in accordance with the present invention.